A Multi-Instrument Study of the Helix Nebula Knots with the<i>Hubble Space Telescope</i>

O’Dell, C. R.; Henney, W. J.; Ferland, Gary J.

doi:10.1086/430803

Cited by 43 publications

(37 citation statements)

References 44 publications

(67 reference statements)

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“…This is similar to the observed changes in clump frequencies in the Helix Nebula, which was recently discovered to be also a multiple-shell planetary nebula (MSPN) (Zhang et al 2012). The optical images of the Helix Nebula do not cover all radii (O'Dell et al 2005) and show a difference in number density at a line about 40% of the radius for the large knots. Matsuura et al (2009) described a low frequency of isolated H 2 clumps in the inner half of the nebula and a strong increase to a few hundred isolated knots per square arcminute in a region they called the inner ring (out to about 70% of the radius).…”

Section: Main Nebulasupporting

confidence: 85%

“…The required amount of ionizing UV photons can be obtained by a clumpy structure of the main nebula, allowing UV photons to escape. Such structures are established for some well-studied PNe such as the Helix Nebula (O'Dell et al 2005;Matsuura et al 2009) or the Ring Nebula (Speck et al 2003;O'Dell et al 2003). In Dalnodar & Kimeswenger (2011), the positions of the spokes of enhanced intensity in the shell of NGC 2438 was shown to be correlated to holes in the main nebula.…”

Section: Introductionmentioning

confidence: 83%

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Ionization structure of multiple-shell planetary nebulae

Öttl

Kimeswenger

Zijlstra

2014

A&A

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Context. In recent times an increasing number of extended haloes and multiple shells around planetary nebulae have been discovered. These faint extensions to the main nebula trace the mass-loss history of the star, modified by the subsequent evolution of the nebula. Integrated models predict that some haloes may be recombining, and thus are not in ionization equilibrium. But parameters such as the ionization state and thus the contiguous excitation process are only poorly known. The haloes are very extended, but faint in surface brightness − 10 3 times lower than the main nebula. The observational limits call for an extremely well studied main nebula, to model the processes in the shells and haloes of one object. NGC 2438 is a perfect candidate to explore the physical characteristics of the halo. Aims. The aim is to derive a complete data set of the main nebula. This allows us to derive the physical conditions from photoionization models, such as temperature, density and ionization, and clumping. These models are used to derive whether the halo is in ionization equilibrium. Methods. Long-slit spectroscopic data at various positions in the nebula were obtained at the ESO 3.6 m and the SAAO 1.9 m telescope. These data were supplemented by imaging data from the HST archive and from the ESO 3.6 m telescope and by archival VLA observations. The use of diagnostic diagrams draws limits for physical properties in the models. The photoionization code CLOUDY was used to model the nebular properties and to derive a more accurate distance and ionized mass. Results. We derive an accurate extinction E B−V = 0.16, and distance of 1.9 ± 0.2 kpc. This locates the nebula behind the nearby open cluster M46 and rules out membership. The low-excitation species are found to be dominated by clumps. The emission line ratios show no evidence for shocks. The filling factor increases with radius in the nebula. The electron densities in the main nebula are ∼250 cm −3 , dropping to ∼10−30 in the shell. We find the shell in ionization equilibrium: a significant amount of UV radiation infiltrates the inner nebula. Thus the shell still seems to be ionized. The spatially resolved CLOUDY model supports the hypothesis that photoionization is the dominant process in this nebula, far out into the shell. Previous models predicted that the shell would be recombining, but this is not confirmed by the data. We note that these models used a smaller distance, and therefore different input parameters, than derived by us.

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Section: Main Nebulasupporting

confidence: 85%

Section: Introductionmentioning

confidence: 83%

Ionization structure of multiple-shell planetary nebulae

Öttl

Kimeswenger

Zijlstra

2014

A&A

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“…This value was calculated from the observed H 2 surface brightness in the Red Spider (Davis et al 2003), and employing the brightness-column density relationship of O'Dell et al (2005). The fractional abundances thus derived, relative to H 2 , are also listed in Table 2.…”

Section: Results and Analysismentioning

confidence: 99%

The Remarkable Molecular Content of the Red Spider Nebula (Ngc 6537)

Edwards

Ziurys

2013

ApJ

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Millimeter and sub-millimeter molecular-line observations of planetary nebula (PN) NGC 6537 (Red Spider) have been carried out using the Sub-Millimeter Telescope and the 12 m antenna of the Arizona Radio Observatory in the frequency range 86-692 GHz. CN, HCN, HNC, CCH, CS, SO, H 2 CO, HCO + and N 2 H + , along with the J = 3 → 2 and 6 → 5 lines of CO and those of several isotopologues, were detected toward the Red Spider, estimated to be ∼1600 yr old. This extremely high excitation PN evidently fosters a rich molecular environment. The presence of CS and SO suggest that sulfur may be sequestered in molecular form in such nebulae. A radiative transfer analysis of the CO and CS spectra indicate a kinetic temperature of T K ∼ 60-80 K and gas densities of n(H 2 ) ∼ 1-8 × 10 5 cm −3 in NGC 6537. Column densities of the molecules in the nebula and their fractional abundances relative to H 2 ranged from N tot ∼ 10 16 cm −2 and f ∼ 10 −4 for CO, to ∼7 × 10 11 cm −2 and f ∼ 8 × 10 −9 for the least abundant species, N 2 H + . For SO and CS, N tot ∼ 2 × 10 12 cm −2 and 10 13 cm −2 , respectively, with f ∼ 10 −7 and 2 × 10 −8 . It was also found that HCN/HNC ≈ 2. A low 12 C/ 13 C ratio of ∼4 was measured, indicative of hot-bottom burning. These results, coupled with past observations, suggest that molecular abundances in PNe are governed principally by the physical and chemical properties of the individual object and its progenitor star, rather than nebular age.

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“…Its origin remains uncertain. Low-Js CO and H 2 observations of the Helix nebula show dense, neutral knots with ionized cometary tails located in the central region of the nebula (Huggins et al 2002;O'Dell et al 2005;Hora et al 2006;Matsuura et al 2007). Matsuura et al (2009) showed that it is in the inner region of the Helix, towards the central star, where tails are observed from the neutral globules probably created by the stellar winds from the central star (Meaburn & Boumis 2010;Matsuura et al 2009;Speck et al 2002).…”

Section: Introductionmentioning

confidence: 99%

Herschelspectral mapping of the Helix nebula (NGC 7293)

Etxaluze

Cernicharo

Goicoechea

et al. 2014

A&A

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Context. The Helix nebula (NGC 7293) is our closest planetary nebulae. Therefore, it is an ideal template for photochemical studies at small spatial scales in planetary nebulae. Aims. We aim to study the spatial distribution of the atomic and the molecular gas, and the structure of the photodissociation region along the western rims of the Helix nebula as seen in the submillimeter range with Herschel. Methods. We used five SPIRE FTS pointing observations to make atomic and molecular spectral maps. We analyzed the molecular gas by modeling the CO rotational lines using a non-local thermodynamic equilibrium (non-LTE) radiative transfer model. Results. For the first time, we have detected extended OH + emission in a planetary nebula. The spectra towards the Helix nebula also show CO emission lines (from J = 4 to 8), [N ii] at 1461 GHz from ionized gas, and [C i] ( 3 P 2 -3 P 1 ), which together with the OH + lines trace extended CO photodissociation regions along the rims. The estimated OH + column density is ∼10 12 −10 13 cm −2 . The CH + (1-0) line was not detected at the sensitivity of our observations. Non-LTE models of the CO excitation were used to constrain the average gas density (n(H 2 ) ∼ (1−5) × 10 5 cm −3 ) and the gas temperature (T k ∼ 20−40 K). Conclusions. The SPIRE spectral-maps suggest that CO arises from dense and shielded clumps in the western rims of the Helix nebula, whereas OH + and [C i] lines trace the diffuse gas and the UV and X-ray illuminated clump surfaces where molecules reform after CO photodissociation. The [N ii] line traces a more diffuse ionized gas component in the interclump medium.

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A Multi-Instrument Study of the Helix Nebula Knots with theHubble Space Telescope

Cited by 43 publications

References 44 publications

Ionization structure of multiple-shell planetary nebulae

Ionization structure of multiple-shell planetary nebulae

The Remarkable Molecular Content of the Red Spider Nebula (Ngc 6537)

Herschelspectral mapping of the Helix nebula (NGC 7293)

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